1. Field of The Invention
This invention relates to a process for preparing L-serine and more particularly, to a process for preparing L-serine from glycine and formaldehyde in the presence of cells of a microorganism or cell-treated product having a serine hydroxymethyl transferase activity by an enzymatic method.
2. Description of the Prior Art
L-Serine is an amino acid which is utilized as medicines, cosmetics and starting materials for chemicals and has now been prepared by chemical synthetic processes or enzymatic processes using glycine as a precursor.
However, the chemical synthetic processes inevitably involve formation of a DL product, with an attendant disadvantage that for the preparation of an L product alone, optical resolution has to be used. On the other hand, the enzymatic process using glycine as a precursor is disadvantageous in the quantity of accumulation, yield, purification and treatment of waste water. Thus, these known processes are not always practically useful processes.
In place of these processes, attention has been recently paid to a process wherein L-serine is enzymatically prepared from glycine and formaldehyde by utilizing serine hydroxymethyl transferase (EC 2.1.2.1, hereinafter referred to as "SHMT").
Moreover, since there is known a process wherein the SHMT activity of microorganisms is improved by gene engineering (Gene, 14, p, 63-72 (1981), Gene 27, p.47 54 (1984)), processes utilizing SHMT obtained from microorganisms are expected to be more industrially advantageous in the future.
As an industrial process of conveniently producing L-serine from glycine and formaldehyde according to an enzymatic process utilizing SHMT, there is known a process wherein microorganisms or their cells having SHMT activity are contacted with a glycine solution and are subsequently used for the reaction with L-serine (Japanese Laid-open Application No. 61-9294). However, it is known that microorganism have a serine decomposition enzymatic activity (hereinafter referred to SD activity) (for example, L-serine dehydratase as described in (1) Shizuta Y. and Tokushige M, Methods In Enzymology 17B, p. 575-580, Academic Press Inc. New York (1971), (2) Burns. R. O. Methods In Enzymology 17B, Academic Press, New York (1971), and (3) Kubota. K. et al, J. Fermentation and Bioengineering 67, (6), p. 391-394 (1988)).
The SD activity presents the following problems in the process for preparing L-serine by the enzymatic method.
(1) Because of the decomposition of produced L-serine, the yield (based on starting glycine) decreases. PA1 (2) The L-serine formation reaction is suppressed by means of the decomposition product of L-serine with a decrease in the amount of accumulated or precipitated serine.
To avoid the above, there is known a process for efficiently producing L-serine by the use of a variant strain of microorganisms wherein the SD activity is reduced. [literature ((1) Kubota K. Agric. Biol. Chem. 49, p. 7-12 (1985)]. However, it is not easy to obtain SD-deactivated variant strains and the appearance of the denatured strains is a problem. Thus, such a process is disadvantageous as a process on an industrial scale.
The suppression of the serine decomposition activity is disclosed in Japanese Laid-open Patent Application Nos. 58-129972 and 58-129975. However, in either application, the bacteria are treated at a specific temperature of 40.degree. to 60.degree. C. within a short time of 10 to 30 minutes. Since the treating time is short, the serine dehydration activity will not be completely deactivated.